Decipher Merchant Seaman Discharge Certificate...help!

Decipher Merchant Seaman Discharge Certificate...help!

I have just stumbled across my great grandfathers discharge certificate from the Merchant Navy and have been trying to find meaning in the information provided with little luck.

I'd like to know what the 'No and Grade of BT Certificate' means (as well as what is written there), and also the ship he worked on, where it went, its purpose etc - I know this information is provided next to the photograpgh as '115813' March 1920. I have searched Miramar ship records which has provided the name of the ship though not much else.